The following describes an arrangement of a conventional liquid crystal display device in which a peripheral driving circuit is formed on one of paired substrates on reference to FIGS. 16 through 18.
FIG. 16 is a plan view showing a substrate with which a peripheral driving circuit is formed, and FIG. 17 is a drawing showing a layout of one picture element. Moreover, FIG. 18 is a cross-sectional view taken along line 18--18 in FIG. 17.
As shown in FIG. 16, a gate driving circuit 32, a source driving circuit 33 and a TFT array section 34 are formed on an insulating substrate 31 which is one of the substrates in the liquid crystal display device. As the insulating substrate 31, a glass substrate, a quartz substrate or the like is used. The gate driving circuit 32 is composed of a shift register 32a and a buffer 32b. Moreover, the source driving circuit 33 is composed of a shift register 33a, a buffer 33b and analog switches 39. The analog switches 39 sample video signals to be inputted from the outside to a video line 38.
A plurality of parallel gate bus wirings 116 which are extended from the gate driving circuit 32 are wired on the TFT array section 34. Moreover, a plurality of parallel source bus wirings 120 extend from the source driving circuit 33 wired on the TFT array section 34 so as to perpendicularly intersect to the gate bus wirings 116. The analog switches 39 are connected respectively to the source bus wirings 120. Moreover, additional capacity common wirings 114 are wired on the TFT array section 34 so as to be parallel with the gate bus wirings 116. Rectangular domains which are surrounded respectively by two gate bus wirings 116, two source bus wirings 120 and two additional capacity common wirings 114 are provided with thin film transistors (i.e. TFT) 35, picture elements 36 and additional capacities 37. The TFT 35 functions as a switching element which electrically connects the picture element 36, the gate bus wiring 116 and the source bus wiring 120. A gate electrode of the TFT 35 is connected to the gate bus wiring 116, and a source electrode of the TFT 35 is connected to the source bus wiring 120.
A drain electrode of the TFT 35 is connected to a picture element electrode of the picture element 36. The picture element 36 is composed of the picture element electrode, a counter electrode provided on a counter substrate which faces the insulating substrate 31, and a liquid crystal layer sealed between the picture element electrode and the counter electrode. Moreover, the additional capacity common wiring 114 is connected to a electrode having the same electric potential as the counter electrode.
The following details the arrangement of the conventional TFT array section 34 in FIG. 16 with reference to FIGS. 17 and 18. A polycrystal silicon thin film 111 which is used as an active layer of the TFT 35 is formed on the insulating substrate 31 so as to have a thickness of, for example, 40 nm-80 nm. Then, a gate insulating film 113 is formed so as to have a thickness of, for example, 80 nm-150 nm by the sputtering or CVD method.
Phosphorus ions (P.sup.-) with concentration of 1.times.10.sup.15 (cm.sup.-2) are implanted into a section 110 (a shaded portion in FIGS. 17 and 18) of the polycrystal silicon thin film 111 where the additional capacity 37 will be formed.
A metal or polycrystal silicon layer with low resistance which is used as the gate bus wiring 116 and the additional capacity common wiring 114 are formed on the gate insulating film 113, and it is patterned so as to have a predetermined shape. As a result, a gate electrode 116a and an additional capacity upper electrode 114a are formed.
Thereafter, in order to determine a conduction type of the TFT 35, phosphorus ions (P.sup.+) with concentration of 1.times.10.sup.15 (cm.sup.-2) are implanted from the upper section of a gate electrode 116a, and a portion under the gate electrode 116a of the polycrystal silicon thin film 111 is a channel section 112 of the TFT 35.
A first inter-layer insulating film 115 is formed on the whole surface of the substrate 31 by using SiO.sub.2 or SiN.sub.X, and contact holes 118 and 119 are provided. Then, the source bus wiring 120 and a piling electrode (drain electrode) 121 are formed in the contact holes 118 and 119 by using metal with low resistance such as Al.
In the same manner as the first inter-layer insulating film 115, a second inter-layer insulating film 124 is formed on the whole surface of the substrate 31 by using SiO.sub.2 or SiN.sub.X, and a contact hole 123 is formed. Then, a picture element electrode 125 is formed by using a transparent conductive film such as ITO. When Al is used for the source bus wiring 120 and the piling electrode 121, for example, in order to bring the piling electrode 121 into ohmic contact with the picture element electrode 125, a barrier metal 126 is formed in the contact hole 123 by using metal such as Ti, TiW, Mo, MoSi.
However, the above-mentioned conventional liquid crystal display device has the following problems.
(1) First Problem
In the above arrangement, since the first and second inter-layer insulating films 115 and 124 are made of inorganic materials, the film thickness is small, i.e. several hundred nm, and the dielectric constant becomes higher than a usual organic material. For this reason, the capacity between the additional capacity common wiring 114 and the other wiring (for example, the source bus wiring 120) becomes large, and the additional capacity common wiring 114 is easily influenced by the other wirings. Therefore, when inorganic materials are used for the inter-layer insulating films 115 and 124, it is not preferable that the additional capacity section is formed so as to greatly overlap the other wirings.
In addition, when the picture element electrode 125 is arranged so as to overlap the gate bus wiring 116 or the TFT 35 on an area connected to the picture element 36, capacity Cgd' is generated between the picture element electrode 125 and the gate bus wiring 116 or the TFT 35. When the TFT 35 is turned off, a voltage drop (.DELTA.V) of the picture element electrode 125 represented by the following equation occurs. EQU .DELTA.V=.DELTA.Vg.times.(Cgd+Cgd')/(Cgd+Cgd'+Cs+C.sub.LC)
(.DELTA.Vg: potential difference between on-state and off-state of the gate, Cgd: capacity between gate and drain of TFT, Cs: additional capacity, C.sub.LC : liquid crystal capacity)
Since a d.c. component is applied to the liquid crystal due to the voltage drop, it is required to apply a bias voltage, for example, to the counter electrode.
In addition, since the additional capacity section does not have a light transmitting characteristic, an aperture ratio is lowered due to the additional capacity section. Moreover, the additional capacity common wiring 114 is formed on the layer where the gate bus wiring 116 is formed, and the additional capacity common wiring 114 does not have the light transmitting characteristic. As a result, the aperture ratio is lowered.
(2) Second Problem
Since the first inter-layer insulating film 115 is made of a inorganic material with a thickness of several hundred nm, disconnection of the source bus wiring 120 occurs due to unevenness of surface in a section where the source bus wiring 120 and the gate bus wiring 116 cross each other.
(3) Third Problem
In the above arrangement, the additional capacity common wiring 114 is formed by using the same material as the gate bus wiring 116, and the gate insulating film 113 just under the wiring 114 is used as a dielectric. Since the gate insulating film 113 is thin and its dielectric constant is high, even if the area is small, large additional capacity can be obtained. However, with this arrangement, when the gate bus wiring 116 is formed by a material with electrically higher resistance than the source bus wiring 120, propagation of a signal tends to be delayed in the additional capacity common wiring 114.
(4) Fourth Problem
In the liquid crystal display device having the above arrangement, a point-at-a time driving method is generally executed. As the other driving method, a line-at-a-time driving method exists, when the line-at-a-time driving is executed, a sampling capacitor for holding a sampled signal for 1 line is required. Moreover, since it is necessary to apply a transfer signal to be used for outputting the signals stored in the sampling capacitor to a hold capacitor all at once, the configuration of the circuit becomes complicated. The point-at-a-time driving does not require these capacitors, and thus a simple configuration of the circuit can be realized. Furthermore, the point-at-a-time driving method is usually used. However, the point-at-a-time driving method requires a higher speed of writing to the picture element through the TFT 35 compared to the line-at-a-time driving method. For this reason, when a-SiTFT is used as the TFT 35, the point-at-a-time driving is not executed, but when p-SiTFT is used, it can be executed.
In the point-at-a-time driving, video signals inputted to video lines 38 shown in FIG. 16 are successively sampled by the analog switches 39 of the source driving circuit 33 so as to be written to the source bus wirings 120. Thereafter, when the TFT 35 is turned on according to a signal from the gate driving circuit 32, the video signal written to the source bus wiring 120 is written to the picture element 36. Therefore, electric charges corresponding to the video signals written to the respective source bus wirings 120 should be securely held at least until the writing to all the source bus wirings 120 is completed.
When the capacity of the source bus wiring 120 is small, since an amount of electric charges written through the analog switches 39 is small, the writing to the picture element 36 is insufficient. As a result, insufficient contrast occurs. More specifically, when a inter-layer insulating film having a low dielectric constant and a large thickness is used, also the capacity formed in a portion where the source bus wiring 120 and another wiring cross each other becomes small. As a result, the capacity of the source bus wiring 120 becomes less and less.
As a method of preventing the insufficient contrast due to an insufficient capacity of the source bus wiring 120, for example, Japanese Unexamined Patent Publication No. 62-178296/1987 (Tokukaisho 62-178296) suggests that a sample hold capacity is formed by an MOS-type capacitor having the same structure as the TFT 35. However, such an MOS-type capacitor is liable to cause a dielectric breakdown due to static electricity during the rubbing treatment which is given to an alignment film on a side where the TFT 35 is provided after the process of manufacturing a substrate. Since the dielectric breakdown of the MOS-type capacitor causes a defect of line because a suitable signal cannot be written to the picture element which is connected to the source bus wiring 120 which is provided with this MOS-type capacitor.
As described in Japanese Unexamined Patent Publication No. 7-175082/1995 (Tokukaihei 7-175082), for example, such a defect of line can be corrected by forming a plurality of sample hold capacity parallel and by cutting off a defective capacity when the dielectric breakdown occurs. However, in this case, a new process for correcting the defect is added.